Energy harvesting technologies for converting ambient sparse energy to power, which is used for the power supply of electronic devices have been attracting attention as technologies according to which sensor nodes for forming a sensor network society of which the market is considered to expand greatly in the future can be realized in a battery-less form.
The future image of the sensor network is a human friendly information-oriented society where innumerable microminiature wireless sensor nodes are provided in a given environment and a massive amount of data from these sensors is processed and fed back to people as information that is useful for making people feel at ease and providing them with a sense of security.
A vast number of wireless sensors are used here, and therefore the wireless sensors are required to be of a self-generating type. Furthermore, it is necessary for the wireless sensors to be so small that the existence of the wireless sensors has no effect on the environment, and thus it is desired to implement a self-generating wireless sensor of which the size is on a scale of millimeters or smaller.
A possible example is an implant-type sensor node that is implanted in an animal or human body so as to generate power from vibration or body heat and transmits a wireless signal containing information on the organism. In order to implement such microscopic electronic devices at a low cost, an ambient power-generating element that is integrated with a semiconductor circuit such as a Si electronic circuit is required.
Ambient power-generating systems include those that use light, vibration or heat. One system that uses light is a solar battery, which can be easily integrated into a semiconductor circuit. As for vibration-powered generation, many reports on Si-based vibration-powered generators using MEMS technology have been issued proving that it is still possible to integrate a vibration-powered generator with a Si circuit.